Field of the Invention
The present invention relates to improvements in pipe connectors, particularly but not exclusively for use in the oil industry, for connecting metal pipe sections of a pipe string. More particularly, the invention relates to improvements in the type of pipe connector described in GB1573945, GB2033518, GB2099529, GB2113335 and GB2138089.
This type of pipe connector includes a tubular pin member having a generally frusto-conical outer peripheral surface and a tubular box member having a generally frusto-conical inner peripheral surface corresponding to the frusto-conical outer peripheral surface of the pin member. In use the two members, each associated with a pipe section, are telescoped together and are axially locked together by inter-engageable annular projections and grooves provided on the peripheral surfaces, the projections and grooves being spaced apart along the two surfaces.
In telescoping the two members together, they are initially telescoped until surface contact is made between crest surfaces of the projections and surfaces between the grooves at least at the ends of the overlapped portions of the surfaces. Hydraulic fluid under pressure may then be supplied between the overlapped parts of the surfaces to expand the box member and/or contract the pin member to permit the members to be fully telescoped together, or the members may simply be pushed together. Pressurised hydraulic fluid is also used to disengage the members by expanding the box member and/or contracting the pin member to bring the projections out of engagement with the corresponding grooves.
In order to reduce the axial extent of the members, projections and grooves may be provided which have relatively small axial extents but this means that, to fully telescope the members together after they have been brought into initial contact, it is necessary to move individual projections past at least one groove before each projection is aligned with the groove in which it is designed to engage. In doing this, there is a risk that the projections and grooves may inter-engage before the members are fully telescoped together and it may then prove impossible to disengage the projections and grooves. To avoid this, some at least of the projections and grooves have different axial extents so that the projections and grooves cannot inter-engage before the members are fully engaged. The arrangement is such that in all intermediate positions of the members, prior to fill engagement and after the frusto-conical surfaces have been brought into initial surface contact, there is contact between the crests of at least some of the projections and surfaces between the grooves spaced apart along the overlying parts of the frusto-conical surfaces.
In the connectors described in the above referred to patent specifications, the projections and grooves have a constant radial height and depth and the crest and root surfaces of the projections and grooves lie on frusto-conical surfaces all of which have the same conicity.
In the connector described in GB2138089, the projections and grooves are spaced from the ends of the frusto-conical surfaces of the members and sealing means for sealing between the surfaces are provided at or adjacent the ends of the surfaces in conjunction with means radially restraining the free ends of the members. As described, the members may be provided at their free ends with axially projecting annular nibs which engage in grooves in the corresponding member to increase sealing at the ends of the frusto-conical surfaces to ensure that there is no loss. or no significant loss, of pressure in the hydraulic fluid used to disengage the members.
However, it is found that, particularly with this type of connector, when disengaging the members using pressurised hydraulic fluid, to ensure that the end projections and grooves at the end of the surfaces of the members disengage completely despite the radial restraint, the radial extent of disengagement of the projections and grooves in the central parts of the surfaces of the members is substantially greater than is needed to disengage these projections from their grooves. This means that the pressure of the hydraulic fluid required to disengage the pin and box members is relatively high. Since the resulting expansion and/or contraction of the members must not exceed the elastic limit of the material of the members, the members have to be made from high tensile steel.